Polyesters such as, for example, polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate, generally referred to as “polyalkylene terephthalates”, are a class of important industrial polymers. They are widely used in thermoplastic fibers, films, and molding applications.
Polyalkylene terephthalates can be produced by transesterification of a dialkyl terephthalate ester with a glycol followed by polycondensation or by direct esterification of terephthalic acid with the selected glycol followed by polycondensation. A catalyst is used to catalyze the esterification, transesterification and/or polycondensation.
Antimony, in the form of a glycol solution of antimony oxide, frequently is used as catalyst in the transesterification or esterification process. However, antimony forms insoluble antimony complexes that plug fiber spinnerets and leads in fiber spinning to frequent shutdowns to wipe spinnerets clean of precipitated antimony compounds. The antimony-based catalysts are also coming under increased environmental pressure and regulatory control, especially in food contact applications.
Organic titanates, such as tetraisopropyl and tetra n-butyl titanates, are known to be effective polycondensation catalysts for producing polyalkylene terephthalates in general, and frequently are the catalyst of choice. However, these catalysts tend to hydrolyze on contact with water, forming glycol-insoluble oligomeric species that lose catalytic activity. These organic titanates may also generate a significant amount of yellow discoloration when used as polyesterification catalysts. Co-catalysts such as zinc, cobalt or manganese have been used with organic titanates to enhance catalyst activity or minimize color, but they still generate some yellow discoloration in the resulting polymer.
JP 2003 306538 and JP 2003 305537 disclose a process for preparing polyesters using as catalyst an aluminum compound, a phosphorus compound and optionally a metal compound selected from Sb, Ge, Ti, Co and Mg compounds. Also, JP 2000 143789 discloses a process for polyesters using as catalyst a titanium compound and a second compound selected from an Al, Ba, Co, Mg, Mn, Sr, Zn, and alkali metal compound, and/or a phosphorus compound.
There is a need for an improved catalyst composition to improve the color of polyethylene terephthalates while maintaining a high catalyst activity and reducing titanium catalyst requirements. Preferably, the catalyst composition is in the form of a stable solution.